Headphones such as circum aural or supra aural types which include ANR are well known. In essence, such headsets include a microphone to sense unwanted noise, and a signal representative of the noise is provided to feedback or feedforward controllers, which then provide a control signal to a driver that incorporates a signal out of phase with the undesired noise. Such devices tend to provide good active noise reduction at low frequencies but have difficulty actively reducing higher frequencies. However, when combined with effective passive insulation provided by a closed ear cup, a broad band noise reduction effect can be realized.
Presently, few active noise reduction earphone solutions exist in the marketplace. The few products that have been developed and commercialised almost all rely on a feedforward active noise reduction configuration.
A feedforward active noise reduction system relies on a reference signal to generate a control response, this reference signal being in some manner related to the signal requiring control.
The best choice of reference signal is then a measure of the ambient noise directly outside of the earphone's passive seal against the ear canal. This reference signal, obtained by way of a microphone transducer, is processed by noise reduction electronic circuitry (filters) to generate an appropriate control response. This is then input into the earphone's speaker, or driver. The circuitry is designed to replicate the dynamic behaviour of the acoustic system between the reference measurement and driver position. All things being equal, the control response, once inverted and output via the earphone's driver, will effect reduction of the noise that has infiltrated the ear canal.
A fixed controller, i.e. one whose parameters are fixed, does not have any measure of its own performance. It relies on a priori knowledge of the disturbance (noise) from the reference signal and the acoustic system.
Thus a fixed or non-adaptive control filter designed for one earphone configuration may represent a less than accurate control filter for another. This may ultimately lead to the creation of an inaccurate control response and poor performance—often amplification of noise (constructive interference) at certain frequencies.
Adaptive filters offer the advantage that the model of the transfer function between the measurement position and speaker is developed in real-time, converging on a best fit approach based on a given cost index. However, performance is often limited by the accuracy of the secondary path model, which again may only be accurate for a single incarnation of the product. Furthermore, adaptive filters often realise poor model accuracy at lower frequencies, where the dynamics of the system maybe of low sensitivity, but where maximum noise cancellation is desired.
A feedback or regulated control configuration alters the control response based on an error signal measured at a position downstream from the driver. This error signal represents the difference between the desired outcome and the measured result. The filtering of the error signal can tailor the performance of the system to provide deep levels of noise cancellation. Since a feedback system is regulated, performance is less sensitive to variations in components and assembly. The increased noise reduction (or depth of noise reduction) available with feedback systems, especially at low frequencies, is a significant advantage over feedforward configurations.
Because connection of the error signal to the control filters creates a feedback loop in the system, the response of a feedback control configuration is susceptible to closed-loop instability. In the context of active noise reduction, instability manifests itself as an uncontrolled ringing. Such a condition is unpleasant and can damage the hearing organ. Instability problems have lead to very few earphones which incorporate active noise reduction systems being successful, commercially viable, consumer products. One such consumer product is described in International Patent Application WO2007/054807 in the name of Phitek Systems Limited and is sold at market as Part No. 2004 ANR Earphone by Phitek Systems Limited. Development of an effective feedback based active noise reduction earphone requires a careful balancing of a number of system parameters.
Engineering an effective and stable feedback-based active noise reduction earphone that provides cancellation over a reasonable bandwidth is a challenging exercise given the limited air volume, low damping and variations commonly experienced in assembling the transducers within a very small acoustic cavity. Placement of the microphone and driver is critical, as is the size and configuration of the acoustic cavity, its venting and damping. To date, the design and manufacture of feedback based active noise reduction earphones has been carefully managed by highly qualified design teams on a product-by-product basis. This makes the design and production process very difficult, time consuming and expensive.